In recent years, the projector market has been growing rapidly along with the use of personal computers. Liquid crystal display elements of the transmission-type and the reflection-type, and DMD display elements which include micromirrors in an orderly array, are known as light valves that modulate light in order to produce image light signals. In particular, an image display device of the reflection-type is suitable to create very small picture elements with high efficiency, and therefore it has gained attention as an image display device for producing a high quality image.
Various projection display devices have been developed that use reflection-type display elements and polarization properties of light beams. For instance, projection display devices that illuminate reflection-type display elements with polarization changes after color separation of the light beams according to wavelengths from a light source by an illumination optical system with a projection optical system that use four polarization-sensitive beam splitters, generally known as a COLORQUAD™, and that project imaging light beams from the reflection-type display elements, are known. FIG. 26 and FIG. 27 are cross-sectional diagrams of prior art Example 1 and prior art Example 2 of such devices. In FIG. 26 and FIG. 27, light beam channels corresponding to each of the three primary colors of light are shown as straight lines, and short intersecting lines and the black round shapes shown on these lines indicating the paths of the three light beams indicate one of two polarization states (S polarized light or P polarized light) of each of the light beams at particular locations in the projection display devices. In the following descriptions, the short intersecting lines are called the first polarization state and the black round shapes are called the second polarization state of the light beams.
Light from a light source (not shown), enters from the bottom as shown in FIG. 26 and FIG. 27 (into COLORQUAD™ 159 as shown in FIG. 26) as three different colors with their polarization states adjusted to be the same (the first polarization state). The light is separated into light beams of the three primary colors in the color quad 159. The light beams are modulated by the three reflection-type, liquid crystal panels 153a, 153b, and 153c, in particular LCOS (Liquid Crystal On Silicon), that are reflection-type display elements with polarization properties for modulating the light beams of the three primary colors with image information. A light beam that contains the image information of all three colors is synthesized and emitted from the COLORQUAD™, projected by the projection optical system 162d, and a full color image is formed on a screen (not shown). Each of the three different light paths shown in FIG. 26 may correspond to any one of the three primary colors blue, green and red in the following description of the operation of the COLORQUAD™ 159.
As shown in FIG. 26, in the COLORQUAD™ 159, four polarization-sensitive beam splitters (which term may hereinafter be abbreviated as PBSs) 170a, 150a, 150b, and 160a are arranged so that their internal polarization-sensitive filters 171, 151a, 151b, and 161, respectively, are aligned in the shape of the letter X. The first PBS is an illumination light beam separation element 170a; the second PBS is an optical path separation element 150a; the third PBS is an optical path separation element 150b; and the fourth PBS is a projection light beam synthesis element 160a. The COLORQUAD™ 159 includes first through third LCOS, 153a, 153b, and 153c, first through fourth wavelength-specific, polarization-transforming elements 143a, 143b, 143c, and 143d, first through third polarizing plates 142a, 142b, and 142c, and quarter-wave plates 152a, 152b, and 152c. Furthermore, in order to improve contrast of the projected image, the following arrangements are made: the polarizing plate 142a adjusts the polarization direction of an incident illumination light beam to the first polarization state; the polarizing plate 142b adjusts the polarization direction of the second color light beam which is incident thereon to the second polarization state; and the polarizing plate 142c adjusts the polarization direction of a projection light beam to the second polarization state. Furthermore, the wavelength-specific, polarization-transforming elements 143a, 143b, and 143d are elements for rotating the direction of linear polarization a specified angle. The wavelength-specific, polarization-transforming elements 143a and 143d transform the second color light beam to the second polarization state from the first polarization state, and the wavelength-specific, polarization-transforming elements 143b and 143c transform the first color light beam to the second polarization state from the first polarization state.
Illumination light beam separation element 170a receives light from the light source (not shown) and interiorly reflects part of the light to optical path separation element 150b and transmits part of the light to optical path separation element 150a, and projection light beam synthesis element 160a receives light from optical path separation elements 150a and 150b to synthesize the light beams to form a projection light beam.
Additionally, in order to achieve improved contrast of a projected image, the following arrangements are made: the polarizing plate 142a adjusts the polarization of the light beam incident on the COLORQUAD™ 159 to a light beam in the first polarization state; the polarizing plate 142b assures the direction of linear polarization of a light beam of a second color is in the second polarization state; and the polarizing plate 142c further adjusts the direction of linear polarization of the light beam projected from the COLORQUAD™ 159 that includes all three colors is in the second polarization state. Furthermore, each of the wavelength-specific, polarization-transforming elements 143a-143d is designed to rotate the direction of linear polarization of each light beam of a particular color a particular amount. The wavelength-specific, polarization-transforming elements 143a and 143d transform the second color light beam to the second polarization state from the first polarization state, and the wavelength-specific, polarization-transforming elements 143b and 143c transform the first color light beam to the second polarization state from the first polarization state.
With further reference to FIG. 26, the first color light beam is reflected within the illumination light beam separation element 170a, transmitted by optical path separation element 150b, and irradiates the LCOS 153a that modulates the first color light beam. The second color light beam is transmitted through illumination light beam separation element 170a and optical path separation element 150a and irradiates the LCOS 153b that modulates the second color light beam. The third color light beam is reflected within the illumination light beam separation element 170a and then is reflected within the optical path separation element 150b, and irradiates the LCOS 153c that modulates the third color light beam.
Furthermore, the first color light beam is modulated with image information for projection at the first LCOS 153a and becomes a light beam of the first polarization state before it is reflected within optical path separation element 150b and transmitted through projection light beam synthesis element 160a for projection. The second color light beam is reflected as a light beam modulated with image information at the second LCOS 153b and becomes a light beam of the first polarization state before it is reflected within optical path separation element 150a and projection light beam synthesis element 160a. The third color light beam is reflected as a light beam modulated with image information at the third LCOS 153c and becomes a light beam of the second polarization state before it is transmitted by optical path separation element 150a and projection light beam synthesis element 160a. Thus, as shown in FIG. 26, the light beams of the three different colors are combined as they are emitted from the COLORQUAD™ 159.
It has also been proposed to use only two PBSs in order to improve the contrast of a projection display device while achieving lower cost, lighter weight, and improved polarization properties over a construction with four PBSs.
FIG. 27 shows a projection display device that uses reflection-type display elements, particularly LCOS 153a-153c, each of which is illuminated subsequent to color separation based on wavelengths of light from the illumination light sources. The projection display device of FIG. 27 uses two PBSs in a manner similar to the projection display device of FIG. 26, and light beams containing the image information from the three LCOS 153a-153c related to different wavelengths are similarly projected through the projection optical system 162d. In the projection display device of FIG. 27 a dichroic mirror 170b initially divides the light beams according to color rather than a PBS such as PBS 170a of FIG. 26 that initially divides the light beams according to polarization state. Similarly, in the projection display device of FIG. 27 a dichroic mirror 160b synthesizes the light beams based on wavelength for projection rather than a PBS such as PBS 160a that synthesizes the light beams according to polarization state.
In the projection display device shown in FIG. 26, a total of four wavelength-specific, polarization-transforming elements 143a-143d are present, each of which is either in the illumination optical system (from the light source to the LCOS) or in the projection optical system (from the LCOS to the projection lens), whereas a total of only two wavelength-specific, polarization-transforming elements are arranged in the projection display device of FIG. 27. However, the angle of incidence properties and wavelength selective properties of the wavelength-specific, polarization-transforming elements are not necessarily satisfactory and may be the main causes of deterioration of contrast and deterioration of image formation performance of the projected image.
Concerning these problems, Japanese Laid-Open Patent Application 2001-100155 describes a projection display device that uses two PBSs and does not include any wavelength-specific, polarization-transforming elements. In this publication, a low cost optical system that uses two PBSs is disclosed for a projection display device that uses reflection-type, liquid crystal display elements. This optical system provides separation of a light beam from a light source or device into plural light beams according to wavelengths by a first dichroic mirror and provides a ninety degree rotation of the direction of linear polarization of one of the separated light beams by a polarization-transforming element. This optical system also uses a second dichroic mirror to further separate one of the previously separated light beams according to wavelengths, as well as to synthesize the other of the light beams previously separated by wavelength at the first dichroic mirror with one of the light beams of a different wavelength separated according to wavelengths at the second dichroic mirror. Subsequently, the light beams of the three different wavelengths illuminate different reflection-type, liquid crystal display elements.
The illumination optical system of this projection display device enables two adjacent reflection-type, liquid crystal display elements to be illuminated with light beams of different wavelengths and different polarization states appropriate for operation with an adjacent PBS without using a wavelength-specific, polarization-transforming element while making the entire device compact and decreasing the number of PBSs required. Two light beams having different directions of linear polarization and different wavelengths are emitted in the same direction toward the PBS from the second dichroic mirror that performs both separation and synthesis of various light beams, and the light beam of the other wavelength is emitted in a different direction.
However, there is a problem with this projection display device in that the polarization-transforming element that changes the direction of linear polarization needs to be in relatively close proximity to the light source (namely, in front of the second dichroic mirror), which makes it necessary for the polarization-transforming element to be relatively large.
Additionally, Japanese Laid-Open Patent Application 2001-100155 includes no description of the polarization element used to adjust the polarization direction in the illumination optical system of the projection display device described. However, in order to make a projection display device with a projected image of satisfactory contrast, a polarization element that adjusts the polarization direction is needed in the optical path. Due to the inability to accommodate the polarization element in the optical path where light beams having different polarization directions are present, the polarization element is placed in this optical system closer to the light source than the second dichroic mirror. However, because this position is closer to the light source, the large size of the polarization element and the deterioration of the polarization properties by placing the polarization element far from the PBSs that are adjacent the reflection-type display elements are concerns. It is preferable that the polarization element be arranged closer to the reflection-type display elements and adjacent to the incident side of a PBS.